Answer
$f=6.7\times 10^{-4} Hz$
Work Step by Step
To find the velocity of the wave, use the formula $$v=f\lambda$$ Solving for frequency $f$ yields $$f=\frac{v}{\lambda}$$ Convert $750km/h$ to $m/s$ to get $$\frac{750km}{1hr} \times \frac{1hr}{60min} \times \frac{1min}{60s} \times \frac{1000m}{1km}=208m/s$$ Substituting known values of $v=208m/s$ and $\lambda=310km=310000m$ yields a frequency of $$v=\frac{208m/s}{310000m}=6.7\times 10^{-4} Hz$$
